Five babies have died since January and the number of confirmed cases is four times higher than the same period last year. Health officials urge anyone in contact with infants to be inoculated.

Whooping cough is now at epidemic levels in California and the state could record the highest number of illnesses and death due to the disease in 50 years, the state's top health official said Wednesday.

Reported cases of whooping cough, also known as pertussis, have quadrupled over the same time period last year, said Dr. Mark Horton, director of the California Department of Public Health. Five infants — all under 3 months — have died, including two in Los Angeles County and one in San Bernardino County.

Whooping cough is a bacterial disease that infects the respiratory system. There have been 910 confirmed cases of the highly contagious disease in California between Jan. 1 and June 15. During the same time last year, 219 cases were confirmed.

An additional 600 possible cases are being investigated.

L.A. County has reported 148 suspected pertussis cases, said Director of Public Health Jonathan Fielding. He urged infants' relatives and caregivers to get a booster shot.

Newborns are most likely to be infected by their siblings and parents. Fielding urged anyone who has a cough to stay away from infants.

Pertussis can make young infants gravely ill very quickly, and health officials recommend that even those with early symptoms be treated with antibiotics. They also say that doctors should strongly consider hospitalizing ill infants in a facility with access to an intensive care unit.

Although many people may think they are still protected from whooping cough because they received inoculations as a child, immunity can begin fading five years after the immunization.

In May, state health officials warned that physicians often don't make a prompt diagnosis of whooping cough in infants because newborns can have deceivingly mild symptoms at first — such as a runny nose with an undetectable or mild cough.